This invention relates to a hydrocracking process and a catalyst for use therein. The invention is particularly concerned with a catalyst containing an ultrahydrophobic zeolite which, when used as a hydrocracking catalyst, selectively yields middle distillates.
Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, and other products known as middle distillates, as well as lower boiling liquids, such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical gas oil comprises a substantial proportion of hydrocarbon components boiling above about 700.degree. F., usually at least about 50 percent by weight boiling above about 700.degree. F. A typical heavy gas oil normally has a boiling point range between about 600.degree. F. and 1050.degree. F.
Hydrocracking is generally accomplished by contacting, in an appropriate reaction vessel, the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. Although the operating conditions within a hydrocracking reactor have some influence on the yield of the products, the hydrocracking catalyst is the prime factor in determining such yields. At the present time middle distillates are not in high demand relative to gasoline in the United States, however, marketing surveys indicate that there will be an increased demand for middle distillates as the year 2000 approaches. For this reason refiners have recently been focusing on midbarrel hydrocracking catalysts which selectively produce middle distillate fractions, such as turbine fuel and diesel fuel, that boil in the 300.degree. F. to 700.degree. F. range.
The three main catalytic properties by which the performance of a midbarrel hydrocracking catalyst is evaluated are activity, selectivity, and stability. Activity may be determined by comparing the temperature at which various catalysts must be utilized under otherwise constant hydrocracking conditions with the same feedstock so as to produce a given percentage, normally about 60 percent, of products boiling below 700.degree. F. The lower the activity temperature for a given catalyst, the more active such a catalyst is in relation to a catalyst of higher activity temperature. Selectivity of hydrocracking catalysts may be determined during the foregoing described activity test and is measured as the percentage fraction of the 700.degree. F.--product boiling in the midbarrel product range of 300.degree. F. to 700.degree. F. Stability is a measure of how well a catalyst maintains its activity over an extended time period when treating a given hydrocarbon feedstock under the conditions of the activity test. Stability is generally measured in terms of the change in temperature required per day to maintain a 60 percent or other given conversion.
As pointed out in U.S. Pat. No. 4,401,556, the disclosure of which is hereby incorporated by reference in its entirety, hydrocracking catalysts containing crystalline aluminosilicate zeolites generally have high activity but relatively poor selectivity for middle distillate products. Because of this, midbarrel hydrocracking catalysts normally employ an amorphous inorganic oxide base containing no zeolitic component. Such catalysts, although selective for middle distillates, are not nearly as active as a catalyst containing a zeolitic component. U.S. Pat. No. 4,401,556 discloses a midbarrel hydrocracking catalyst containing an ultrahydrophobic crystalline aluminosilicate zeolite which catalyst possesses both high activity and high selectivity for producing middle distillates. According to the patent, the selectivity of the ultrahydrophobic zeolite component is abnormally high while the activity and stability of the zeolite are not impaired when compared to other known zeolite supports. The ultrahydrophobic zeolite is prepared from a Y type zeolite starting material having a silica-to-alumina mole ratio of from about 4.5 to about 6.0 and a sorptive capacity for water vapor of at least 6 weight percent at 25.degree. C. and a p/p.degree. value of 0.10 by calcining the zeolite powder in an environment comprising from 0.2 to about 10 atmospheres absolute of steam at a temperature ranging from 725.degree. C. to 870.degree. C. for a period of time sufficient to reduce the zeolite's sorptive capacity for water vapor to less than 5 weight percent at 25.degree. C. and a p/p.degree. value of 0.10.
Midbarrel hydrocracking catalysts have been prepared using one of the ultrahydrophobic zeolites disclosed in U.S. Pat. No. 4,401,556 by subjecting the zeolite to an ammonium exchange and then mixing the ammonium-exchanged ultrahydrophobic zeolite with an inorganic refractory oxide component and an alumina binder material. The resultant mixture is then extruded through a die to form extrudates which are dried at 120.degree. C. and subsequently calcined in air at 900.degree. C. The calcined extrudates are then impregnated with a solution of nickel and tungsten components, dried and again calcined in air. It has now been surprisingly found that different batches of hydrocracking catalysts prepared in accordance with the above-disclosed procedure have varying selectivities for middle distillates, some of which selectivities are relatively low. The commercial use of a midbarrel hydrocracking catalyst with lower than desired selectivity for middle distillates will result in a loss of the desired middle distillate product.
Accordingly, it is one of the objects of the present invention to provide a midbarrel hydrocracking catalyst containing an ultrahydrophobic zeolite, and a method for preparing such a catalyst, which is useful in hydrocracking and has high selectivity for middle distillates, which selectivity does not substantially vary from one batch of catalyst to another. This and other objects of the invention will become more apparent in view of the following description of the invention.